High temperature conducting (HTS) magnets, such as HTS magnets formed from rare earth barium copper oxide (REBCO), bismuth strontium calcium copper oxide (BSCCO), yttrium barium copper oxide (YBCO) and magnesium diboride (MgB2), are designed to produce high magnetic fields to store large amounts of magnetic energy during operation. However, the stored energy in the HTS magnet may subject the magnet to a failure mechanism referred to as a “quench”, in which the stored energy is suddenly converted into heat accompanied by the presence of large electrical voltages. A quench typically occurs in an HTS magnet when a conductor transitions from the superconducting state to the normal state is some region of one of the magnet coils. In the normal state, the non-superconducting region of the conductor exhibits an increasingly large electrical resistance, relative to the super-conducting state, resulting in excessive heating of the magnet. The excess temperature and voltage in the windings of the HTS magnet cause by a quench condition can potentially damage the magnet.
In the event of a quench condition in a large superconducting magnet, the current in the superconducting magnet must be rapidly reduced to prevent damage to the magnet resulting from a peak hot-spot temperature or from a mechanical strain. Any method to reduce the magnet current by altering the magnet current through external means, such as a resistor, would involve large voltages, which is undesirable. Alternatively, by warming a large portion of the superconductor to above critical temperature (Tc), the resulting resistance may be distributed throughout the superconducting device, which can dramatically reduce the peak voltages experienced by a quenching coil.
It is known in the art to integrate a set of embedded heaters into the magnet windings to warm the superconductor, thereby distributing the thermal energy throughout the magnet. Additionally, a method is known in the art which relies on AC coupling, commonly referred to as Coupling Loss Induced Quench (CLIQ). In CLIQ, the magnetic energy is distributed by driving an imbalance in the transport current between two or more sections of the magnet. While these known techniques are adequate for low temperature superconducting magnets, they provide marginal quench protection for recently developed high temperature superconductors and are not effective in rapidly distributing a large amount of thermal energy to the windings of an HTS magnet. Additionally, in CLIQ based quench protection systems, to increase the available energy, either the capacitance of the system must be increased, which slows down the frequency, or the voltage must be increased, which results in safety concerns.
Protection of HTS magnets for reliable operation has proven to be a challenge, particularly in Rare Earth Barium Copper Oxide (REBCO) superconductors, due to the large amount of energy that is required to get enough of the current into the metallic stabilizer to properly distribute the magnetic energy and to minimize peak hot-spot temperatures during a quench condition.
Accordingly, what is needed in the art is an improved quench protection system for high temperature superconductors (HTS).